If polyurethane foams are to be used for certain purposes, it is essential for them to have high fire resistance. This resistance is measured in various tests for flammability which may vary according to the particular purpose for which the material is to be used and may in many cases be particularly directed towards certain specific end uses. The MVSS-302 Test, the Federal German Railway Test, the Lufthansa Test (FAR 25 853) and the Small Burner Test according to DIN 53 438 can be viewed in this light.
Questions concerning the use of polyurethanes to meet special fire requirements have been studied with the greatest care, as is evident from the copious literature and many publications on this subject.
Of particular interest are the esters of acids containing phosphorus and the derivatives of these acids as well as halogen compounds and in particular compounds of bromine. The most important flame-retardants to be mentioned in this connection are trichloroethylphosphate and its higher homologues, tricresylphosphate, derivatives of aminomethylphosphonic acid ester, tris-(2,3-dibromopropyl) phosphate, and butene- and butane-diols containing bromine and the relatively high molecular weight oxirane addition products thereof. Melamine and its phosphoric acid derivatives are also known as flame-retardants.
The preparation of foams using amides or esters of divalent or higher valent mononuclear aromatic acids which amides or esters have molecular weights below 400 and which esters or amides contain 2 to 4 hydroxyl groups, optionally in admixture with additional polyols which are free from amide and ester groups and which have an equivalent weight below 200 and a functionality of from 2 to 8 has been disclosed in German Offenlegungsschrift No. 1,745,134. Such amides of aromatic polycarboxylic acids, however, are highly viscous or vitreous masses which are not easily dissolved or dispersed in polyols. At best, such amides can be adequately dispersed only in foam formulations containing water as blowing agent.
The use of bis-(.beta.-hydroxyalkyl)-diamides of aliphatic dicarboxylic acids partly or completely propoxylated on the hydroxyl groups for the production of rigid or semi-rigid polyurethane foams has been described in Austrian Pat. No. 294,429.
The known flame-retardants have various disadvantages in use. Apart from possible harmful physiological effects (especially of .beta.-halogen esters of phosphorus) another known disadvantage of this class of substances is the frequently insufficient resistance to hydrolysis. This is particularly inconvenient when processing the polyurethane starting components since, as is well known, the addition reaction is catalyzed by compounds containing tertiary amino groups. Since the instability of these flame-retardants is even worse in the alkaline region, formulations of such raw materials can only be stored for a limited time. Sometimes it is found that even formulations which are prepared immediately before processing show signs of changes in activity within a few hours. When such changes occur, they are generally difficult to correct and the parts produced are discarded as rejects since subsequent catalysis is very problematic. In addition to the serious disadvantages of these flame-retardants already described, their plasticizing effect should be mentioned, which results in a considerable lowering of the heat distortion temperature. Added to this is the loss in activity which occurs in the course of time due to the volatility of the compounds used. In motor vehicles, for example, this diffusibility of the substances manifests itself in the repeated formation of deposits on glass surfaces, especially on windshields (fogging).
It was an object of the present invention to provide compounds having a flame-retarding action for use as reactive flame-retardants for polyurethane foams (integral skin or rigid), which flame-retardants should be free from the disadvantages known in the art, namely insufficient solubility or dispersibility in the starting materials, insufficient storage stability of the mixture containing the flame-retardants, softening or plasticizing effects of the flame-retardants, inadequate thermal stability of the foam under load, effects of diffusion and bleeding of the flame-retardant and thus loss in activity, and finally, the release of hydrogen halides in cases of total combustion.